In numerous publications on the subject of the preparation of propylene oxide, there are only a few which are concerned with the purification of the solvent which is in most preferred cases methanol, and the re-use of this solvent in the epoxidation reaction.
EP 1 359 148 A1 describes a process in which, prior to separation of methanol from a mixture which essentially comprises methanol and water, the mixture is subjected to hydrogenation, and the resulting (hydrogenated mixture) is distilled to purify and afterwards re-use the solvent. As a preferred embodiment, this document of the prior art describes a process in which, after hydrogenation and prior to distillation, the pH of solvent stream to be purified is adjusted to values of below 7 using an acid such as sulfuric acid.
EP 1 424 332 A1 describes a process in which the crude propylene oxide resulting from an epoxidation reaction and to be purified is admixed with a compound containing an unsubstituted NH2 group and which is capable of reacting with acetaldehyde. This mixture is subjected to extractive distillation to obtain purified propylene oxide. This document of the prior art also discloses an embodiment according to which the crude propylene oxide stream is mixed with an aqueous alkaline solution before feeding it to the extractive distillation column. According to the specific disclosure of this document, addition of this aqueous alkaline solution leads to the conversion of methyl formate to methanol and formate by hydrolysation.
WO 02/14298 A1 describes a process for the continuous preparation of an olefinic oxide by direct oxidation of an olefin with hydrogen peroxide which, among several other steps, a step (c) in which the tail product of a distillation zone and an aqueous basic solution are fed into a decomposition zone, wherein this zone contains a decomposition catalyst which decomposes hydrogen peroxide comprised in the tail product to oxygen and water. The aqueous basic solution is added in order to control the pH of the catalytically driven hydrogen peroxide decomposition reaction which must have values greater than 10.
WO 99/26937 A1 relates to removing oxygenate impurities from an organical chemical feed by treatment with an aqueous reducing agent and an aqueous base, accompanied by separation. It is disclosed that it is preferred that the aqueous base is introduced in a molar equivalent excess relative to the level of oxygenate impurities present. As preferred reducing agents, sulfurous acid, sodium hydrogen sulfite, sodium sulfite, sodium metabisulfite, sodium borohydride, and potassium borohydride and mixtures thereof are described. According to the most preferred embodiments, the streams which are treated by reducing agents and/or aqueous bases essentially consist of isobutane, propylene, propylene oxide and mixtures thereof, wherein this steams contain the oxygenate impurities. Therefore, for example, these streams are treated with aqueous base in order to remove the impurities from propylene oxide.
U.S. Pat. No. 4,691,034 relates to the purification of propylene oxide by treatment with calcium hydroxide in glycerol or sugar water. From such process, propylene oxide is obtained which is essentially free of methyl formate. Glycerol or sugar water are used as solubilizing agents to improve the solubility of calcium hydroxide. As in WO 99/26937 A1, the stream to be treated with base essentially consists of propylene oxide to be purified.
U.S. Pat. No. 3,477,919 which is cited in U.S. Pat. No. 4,691,034 relates to the use of calcium hydroxide slurry to remove methyl formate contaminant from propylene oxide prepared by the epoxidation of propylene with tertiary butyl hydroperoxide. In this process, the calcium hydroxide is used to produce methanol and the calcium salt of formic acid from methyl formate.
U.S. Pat. No. 3,816,478 relates to the purification of a material containing aldehyde impurities. Such materials are organic liquids containing small amounts of water. Treatment is carried out by passing the contaminated stream through a bed of sodium bisulfite as reducing agent.
US 2004/0068128 A1 discloses a process for the manufacturing of an epoxide which comprises reducing, preferably hydrogenating a mixture comprising alpha-hydroperoxy alcohols which are converted to propylene glycols. The reduced mixture is then subjected to a further step in which a solvent contained in the mixture is separated and, if desired, recirculated into the process.
It is an object of the present invention to provide a novel process for the epoxidation of propene which comprises the use of methanol as solvent and the recirculation of this solvent, which process allows for decreasing the content of the methanol regarding carbonyl compounds, particularly acetaldehyde, the methanol having a decreased content of carbonyl compounds preferably being recirculated as solvent in the epoxidation of propene.
It is yet another object of the present invention to provide work-up stages for reaction mixtures obtained from epoxidation reactions which allow for recirculation of a pure solvent used in the epoxidation reaction.
It is a further object of the present invention to provide a process for the purification of mixtures containing methanol and carbonyl compounds.